The present invention relates to paper machines and methods for operating the same.
In particular, the present invention relates to the initial part of the drying section of a paper machine where a web undergoes an initial drying action upon being transported away from the press section of the paper machine.
The drying section of the paper machine is in the form of a multiple cylinder dryer having a pair of staggered rows of drying cylinders which may be disposed one above the other. The first group of drying cylinders of these rows where the web is initially received in the drying section serves to guide a first endless fabric, in the form of a suitable wire or felt, with the cylinders of one row being situated within the closed loop of this first endless fabric while the cylinders of the other row are situated outside of the loop to form outside cylinders. At the inside cylinders the endless fabric is situated between the latter cylinders and the web, and it is at this location that the particular problems are encountered as set forth below. With respect to the outside cylinders, the web is situated between the latter and the above first endless fabric. The web is to be understood as being a full-width paper web, which is to say a web having a width almost equal to that of the cylinders of the drying section.
As the operating speeds and operating widths of paper machines increase, a serious problem is encountered in connection with web breaks which occur at the initial part of the drying section. As a result of this problem, certain multiple cylinder dryer constructions have been proposed in the prior art wherein the so-called closed conduction has been implemented. In other words the constructions are such that the web is conveyed without any open draw.
With respect to the state of the prior art pertinent to the present invention, reference may be made to U.S. Pat. Nos. 2,874,997, 2,091,905, 3,503,139, and 3,576,078, as well as to the German Offenlegungsschrift No. DOS 2,365,458.
In U.S. Pat. No. 2,874,997 there is a multiple cylinder dryer having a closed web conduction, or in other words web transporting without an open draw, with both contact drying and perfusion drying being combined by arranging the cylinders of one row of the multiple cylinder dryer in such a way as to be cylinders which admit a drying gas flow through the walls thereof, while arranging adjacent to the latter cylinders special hoods from which a drying gas flow is conducted through the web, the supporting wire, and the foraminous cylinder shells.
A similar construction is disclosed in the above German publication No. DOS 2,365,438. In this latter disclosure there has been provided a narrow web-introduction strip or tail which is situated on the web which passes over the upper row of the drying cylinder group, with this strip or tail being moved by way of a special structure from the marginal area of the cylinders so as to be on top of the web.
U.S. Pat. No. 2,091,805 shows a multiple cylinder dryer wherein there is also a closed web conduction so that there are no free or open draws of the web. According to this patent also, in those embodiments where the web passes from one cylinder row to the other in zig-zag fashion, supported by a wire, one of the rows of drying cylinders consists of suction cylinders having a permeable shell through which a fluid such as air can flow.
With respect to U.S. Pat. No. 3,503,139, there is also disclosed a multiple cylinder dryer wherein the web passes from one row of the cylinder group to the other while supported by a wire. In this connection the temperature of the cylinders of the upper row has been provided at a value higher than usual because on these particular cylinders the wire or felt is interposed between the web and the surface of the drying cylinders.
U.S. Pat. No. 3,576,078 discloses a multiple cylinder dryer having a pair of wires utilized for directing the web through the entire drying cylinder group. With respect to the drying section of this reference, this particular drying section has perfusion cylinders. However, the web is compelled to travel both at the upper as well as the lower cylinders between two wires at all times, so that there is the serious drawback that at the points where the path of the wires is curved there are necessarily speed differentials between the wires and the web, causing unavoidable friction with the web and giving rise in turn to detrimental dust formation as well as to impairment of the quality of the web surface.
Thus, with the exception of this last-mentioned U.S. patent, where a pair of wires are used throughout, it is essential with the other prior art constructions to utilize in at least one of the cylinder rows drying cylinders which have permeable shells and in connection with which there are provided suction and/or over-pressure chambers.
The fact is that cylinders having foraminous and thus permeable shells, as well as the suction or pressure equipment associated therewith, are comparatively expensive.
It is to be noted that perfusion cylinders are designed not so much for drying purposes. Their main purpose is to eliminate the effects of centrifugal force and the detachment and excessive distention of the web resulting therefrom when the web curves on the surface of the drying cylinders without any other support.